1. Field of the Invention
The present invention relates to an objective lens and an optical pickup device which perform at least one of the reading and reproduction of information from and into an optical disk or an optical card.
2. Description of the Related Art
FIGS. 8 and 9 are diagrams each showing the optical configuration of an optical pickup device of a related art, in which FIG. 9 is a diagram seen from the side surface of the optical configuration of FIG. 8.
In FIGS. 8 and 9, a laser light ray 102a emitted from a semiconductor laser 101 is reflected by a flat plate beam splitter 103, then incident into a collimator lens 104 as a light ray 102b and formed into a parallel light ray 102c after passing through the collimator lens. A part 102d of the laser light 102a passes through the flat plate beam splitter 103 and is received by a laser emission light monitor sensor 105. The flat plate beam splitter 103 separates an incident light into a reflection light reflected thereby and a transmission light passing therethrough and is mostly designed so as to set a ratio between the refection light amount and the transmission light amount to 8:2, for example. The parallel light ray 102c is reflected by a rectangular mirror 106 and incident into an objective lens 107 as a parallel light ray 102e. The parallel light ray 102e is formed into a converged light ray 102f by the objective lens 107 and irradiated on an optical disk 108 as a spot 102g. Information is recorded on and reproduced from the optical disk 108 by means of the spot 102g. 
A reflection light ray of the spot 102g reflected by the optical disk 108 propagates in a direction opposite to that of the aforesaid optical path in a manner of 102f→102e→102c→102b. Supposing that the flat plate beam splitter 103 is designed so as to have a ratio between the refection light amount and the transmission light amount to 8:2 as described above, for example, 20% of the entire reflection light ray from the optical disk 108 passes through the flat plate beam splitter 103 (a light ray 102h of FIGS. 8 and 9) and is incident into a cylindrical lens 109. The cylindrical lens 109 is a lens which acts as a concave lens as to a light ray propagating along the drawing sheet of FIG. 8 but does not act as a lens as to a light ray propagating along the drawing sheet of FIG. 9. Thus, the light ray 102h is converted into a light ray 102i having astigmatism. By using the light ray 102i having the astigmatism and a light receiving sensor 110, a focus error signal, a tracking error signal and an RF signal can be obtained as well known as the astigmatism method in the field of the optical head for an optical disk. Since the theory for generating the focus error signal, tracking error signal, RF signal etc. according to the astigmatism method is disclosed in (Patent Document 1), the detailed explanation thereof will be omitted here.
Patent Document 1: JP-A-2001-160223
In an optical disk driving apparatus which is one of the application fields to which the invention is applied, the optical disk driving apparatuses are classified into a half height driving apparatus with a thickness of 42 mm, a slim driving apparatus with a thickness of 12.7 mm and a ultra slim driving apparatus with a thickness of 9.5 mm when classified in view of the thickness of the optical disk driving apparatus. Further, the optical disk driving apparatuses are classified into a CD type, a DVD type and a BD (compliant with a blue ray disk) type in view of the functions of the optical disk driving apparatus.
The invention makes it be possible to easily thin the apparatus in the slim driving apparatus and the ultra slim driving apparatus. In order to thin the apparatus, it is necessary to thin the reflection mirror 106 and the objective lens 107. However, in the BD driving apparatus (the driving apparatus compliant with a blue ray disk) expected to be spread in the future, since the standard thereof defines to use an objective lens with a numerical aperture of 0.85, the thickness of the center of the objective lens also becomes necessarily larger than that of the CD type and the DVD type, so that there arises a large problem that it is very difficult to thin the apparatus. For example, when the lens thickens is compared as to commercial lenses having the same focal distance of 2 mm, the thickness for each of the CD type and the DVD type is 1.147 mm but the thickness for the BD type is 3.262 mm which is almost three times as large as that of each of the CD type and the DVD type. Thus, in the optical disk apparatus compliant with a blue ray disk, there is a problem that the entire thickness of the optical disk apparatus must be large by an amount corresponding to the larger thickness of the objective lens.
FIGS. 10 and 11 are diagrams each showing an example of the configuration of an optical pickup device of a related art proposed in order to solve the aforesaid problem. FIGS. 10 and 11 are diagrams each showing the optical configuration of the optical pickup device of the related art, in which FIG. 11 is a diagram seen from the side surface of the optical configuration of FIG. 10. The optical pickup device of the related arts shown in FIGS. 10 and 11 is arranged in a manner that the objective lens 107 used in FIGS. 8 and 9 are replaced by a flat plate Fresnel lens 111 in order to thin the apparatus.
FIG. 13 is a diagram showing the relation between the numerical aperture and the pitch of the diffraction grating of the Fresnel lens, in which the abscissa represents the numerical aperture (NA) and the ordinate represents the pitch of the diffraction grating. Further, in this figure, the pitches of the diffraction grating of the Fresnel lens are calculated under the condition that the numerical aperture is 0.85 and the wavelength of a light ray is 405 nm in FIGS. 10 and 11. Furthermore, although, in FIG. 13, the refractive lens is replaced by the Fresnel lens in order to simplify the calculation, the calculation results are almost same as the case of the refractive lens which is used actually.
As clear from FIG. 13, the pitch becomes abruptly small at the outer periphery of the lens (an area having a large numerical aperture (NA)) as compared with the center portion of the lens (an area having a small numerical aperture (NA)) and becomes about several microns at almost numerical aperture (NA) of 0.85. Thus, although an enlarged diagram of the flat plate Fresnel lens in the optical pickup device of the related art is shown in FIG. 12, this Fresnel lens 111 has a problem that the manufacturing thereof is difficult or a high-grade fine processing technique is required in order to minimize the pitch of the diffraction grating. According to the current technique, it is almost impossible to manufacture the aforesaid Fresnel lens 111.